1. Field of the Invention
The present invention relates to a reflector, a method for producing the same, and a reflection type liquid crystal display incorporating the same. More particularly, the present invention relates to a method for producing a reflector in which convex/concave portions are formed on a substrate surface over a plurality of exposure steps, a reflector produced by such a method and a reflection type liquid crystal display incorporating the same.
2. Description of the Related Art
A reflector for use in a reflection type liquid crystal display capable of producing a paper white display includes a large number of convex/concave portions on a surface thereof in order to reflect and scatter light incident thereupon. There are various methods for forming such convex/concave portions, including: forming the convex/concave portions directly on a substrate surface using a sand blast method or an etching method; first forming a thin inorganic oxide film or a metal film on a substrate surface and then partially etching away the thin film; and forming concave portions of an inorganic material on a substrate surface by a plasma CVD method. In order to form the convex/concave portions with high precision and high reproducibility so that the designed reflection characteristic is realized precisely and reproducibly, it is preferable to first form a thin photosensitive organic film on a substrate and then pattern the formed film by a photolithography process. In the exposure step, an exposure machine (projection aligner) is used such as a step and repeat method type exposure (hereinafter referred to as a stepper exposure machine) or a large batch exposure machine.
However, such conventional methods have the following problems. While a large batch exposure machine can expose a large area at once, it has a relatively large variation in irradiation intensity and parallelism of light over a plane to be exposed to the light. A reflector produced by such a large batch exposure machine has a brightness which is not uniform over the screen (e.g., reflection is bright in the vicinity of the exposure center and dark in the peripheral region) and thus has not been in practical use. In view of this, a stepper exposure machine typically has been used in which source light is substantially collimated by a lens system so as to reduce the in-plane variation. However, referring to Figure 1A, the maximum area (indicated by "11") which can be exposed at once by such a stepper exposure machine is only about 6 inches in diameter. Thus, the largest square region 10 on which convex/concave portions can be formed at once has a diagonal line of up to about 6 inches.
In order to expose a larger region such as a rectangle in which a diagonal line connecting opposite corners is about 6 inches or larger with a stepper machine having an exposure area of 6 inches in diameter, for example, a first region 12a can be exposed in the first step (the largest area which can be exposed is denoted by reference numeral 13a, and the exposure center by 14a), and a second region 12b can be exposed in the second step (the largest area which can be exposed is denoted by reference numeral 13b, and the exposure center by 14b), as illustrated in FIG. 1B. However, even when a stepper exposure machine is used, some in-plane light beam variation (or "image distortion", where light beam parallelism is high in the center portion and low in the peripheral region) still exists. Therefore, the obtained reflector causes interference fringes, whereby a "junction (boundary)" 15 between the first and second regions is observed.
In order to reduce the in-plane light beam variation of the stepper exposure machine, the regions 12a and 12b to be exposed during each exposure step can be reduced. However, this solution increases the number of exposure steps required and thus the number of alignment steps associated therewith, thereby considerably decreasing the production efficiency.